1. Field of the Invention
The present invention relates to a system technique which utilizes a boosting active filter which functions as a DC power-supply circuit for a load, and a technique for controlling the active filter.
(1) First, the present invention relates to a system which uses a boosting active filter as an AC-DC converter or a DC-DC converter which provides (a) control of DC output voltage, (b) power-factor improvement of the power supply, and (c) a measure to counter power-supply harmonics, and a device for controlling the active filter.
More specifically, the present invention relates to a technique which is used for such an active filter to certainly limit the variation of the DC output voltage within a given range around the set value over the entire load range including a situation in which the load condition largely changes from the no-load or light load to the rated load or heavy load and the opposite situation.
(2) The present invention further relates to a system for supplying a DC output voltage to the load by using a boosting active filter as an AD-DC converter or a DC-DC converter which provides (a) control of variable DC output voltage, (b) power-factor improvement of the power supply, and (c) a measure to counter power-supply harmonics, and a device for controlling the active filter.
More specifically, the present invention relates to a boosting active filter system having an overvoltage preventing control function which can certainly control a variable DC output voltage within a given range around a certain set value over the entire load range including a situation in which the load condition largely changes from the light load or no-load to the rated load or heavy load and the opposite situation, and which can follow a new set voltage even if the set voltage is changed, and to a device for controlling the active filter.
2. Discussion of the Background
The basic circuit configurations of the boosting active filters have conventionally been studied and known. For example, Japanese Patent No.2624793 (Japanese Patent Publication No.7-89743) discloses a basic circuit configuration thereof. FIG. 13 shows the structure of the main circuit corresponding to the basic circuit.
As shown in FIG. 13, the main circuit includes converter diodes D1 to D2 forming a full-wave rectifying circuit, a DC reactor L, a switching element Q1, a rectifier diode D5, a load capacitor C, and a load R.
Some methods for controlling this main circuit have also been suggested and used in practice. Many ICs specialized to realize the control systems are commercially available. FIG. 14 shows an example of circuit configuration of the entirety of an actually used active filter system.
As shown in FIG. 14, this system generally includes a single-phase AC power supply 1, a noise filter 25, a main circuit (active filter) 100, a control circuit 200P for controlling the main circuit 100, and a load 10. Among these components, the main circuit 100 includes diodes 2 to 5 forming a single-phase full-wave rectifying circuit, a reactor 6, a switching element 7, a rectifier diode 9, a load capacitor 11 connected to the load 10 in parallel, and a current detecting resistor 12. The control circuit 200P includes resistors 29 to 32 for dividing the DC output voltage v.sub.o, a voltage setter 13 for indicating a set value of the load voltage, a voltage amplifier (a differential amplifier) 14 for amplifying the difference between the load voltage and the voltage set value, and a multiplier 15 receiving a current obtained by applying full-wave rectification to the AC power-supply current at its one input and an output from the voltage amplifier 14 at its other input, for calculating and outputting the product.
The control circuit 200P further includes a current amplifier 16 for amplifying the difference between the AC power-supply current detected by the current detecting resistor 12 and the output from the multiplier 15, an oscillator 17 for generating a signal having a triangular waveform, and a comparator 18 having its one input connected to the output of the oscillator 17 and its other input connected to the connection point "a" between a constant-current source 26 and the anode of a diode 28 having its cathode connected to the output of the current amplifier 16.
The comparator 18 makes comparison between its two input signals to output a control signal at a given frequency, whose waveform has a duty factor varying in accordance with the result of the comparison. The control signal is amplified by a gate driver 19 and then applied to the gate electrode of the switching element 7, and the switching element 7 turns on and off in correspondence with the duty factor of the control signal.
When the switching element 7 is on, the current inputted from the AC power supply I flows in the closed circuit including the rectifier diodes 2 to 5, the reactor 6, the switching element 7 and the current detecting resistor 12. After that, when the switching element 7 turns off, the current flowing through the reactor 6 cannot flow to the switching element 7, and it flows to the load 10 and the load capacitor 11 parallel-connected to the load 10 through the rectifier diode 9. Then the load capacitor 11 is charged and the DC output voltage or load voltage v.sub.o rises. As a result, in the period in which the switching element 7 is in the OFF state, the rectifier diode 9 is reverse-biased by the charge voltage of the load capacitor 11 and the current does not flow. Thus, in the OFF period of the switching element 7, electric charge is discharged from the load capacitor 11 to the load 10 and current is supplied to the load 10.
The control circuit 200P further includes a soft start DC power supply 23 which directs the main circuit to make soft start immediately after an activation command signal ON is inputted and activation after the soft start, and a start switch 22 which connects the DC power supply 23 to the cathode of the diode 27 series-connected to the constant-current source 26 in response to the input of the activation command signal ON. When the start switch 22 operates to apply the power-supply voltage of the soft start DC power supply 23 to the point "b," the control circuit 200P and therefore the entire system including the active filter is started. To stop the entire system, an activation stop command signal OFF is applied to control the start switch 22 to the ground potential.
The control circuit 200P also has a hysteresis comparator 20 for the object described later; when the voltage obtained by dividing the DC output voltage v.sub.o at the resistors 31 and 32 exceeds an overvoltage determining reference level (trip operation point) determined by the set value 24, the comparator 20 outputs an overvoltage detect signal to a flip-flop 21 to latch the flip-flop 21. In response, an overvoltage detect signal F.sub.o is outputted to the outside.
When detecting the output of the signal F.sub.o, an external microcomputer 500 generates the activation stop command signal OFF and sends it to the start switch 22 to switch the start switch 22 to the ground potential.
In this way, the conventional system is constructed so that the entire system is stopped by external control when the voltage obtained by dividing the DC output voltage across the load terminals at the two resistors 31 and 32 rises to or over a level corresponding to the overvoltage (the trip operation point).
In this conventional circuit, when the load 10 changes from the rated heavy load condition to the light load or no-load condition, the output voltage v.sub.o of the active filter rises higher and higher by the mechanism described below over the range in which the output voltage can be controlled constant.
The "heavy load condition" means a condition in which the load operates while satisfying the 100% condition with respect to the rating, and the "light load condition" is defined as a condition in which the load operates in 5% to 10% condition with respect to the rating. The "no-load condition" corresponds to 0% of the rating.
The rise of the output voltage v.sub.o of the active filter causes the problem of deteriorating the performance of the load capacitor 11 (e.g. an electrolytic capacitor) on the basis of the relation between the rise value and the breakdown voltage of the load capacitor 11 connected between the output terminals.
For a countermeasure, the system adopts the method in which the output voltage is detected and, when the output voltage rises to an overvoltage being more than or equal to a certain constant value, the control circuit itself is forced to the OFF state by an external microcomputer to stop the active filter operation so as to hinder the rise of the output voltage. The overvoltage detecting circuit shown in FIG. 14 corresponds to the portion for realizing this system, which includes the hysteresis comparator 20, the flip-flop 21, and the two resistors 31 and 32.
Now the mechanism in which the output voltage v.sub.o gradually rises in the light load or no-load condition will be described in detail.
In the circuit diagram FIG. 14 showing the conventional active filter system, the frequency of the oscillator 17 is selected to about 10 kHz to 50 kHz. It is determined on the basis of the switching capability of the switching element 7 and the frequency characteristic of the reactor 6. In FIG. 3, (A) shows an example of the output waveform of the oscillator 17. In (A) of FIG. 3, the oscillation frequency is 20 kHz, and therefore the period is 50 .mu.s. The oscillator 17 itself is designed so that its triangular wave has a valley voltage (a first level) at 2.0 V and a peak voltage (a second level) at 3.0 V.
This triangular-wave output signal is supplied as one input to the comparator 18 as stated before. The connection point a between the diode 28 connected to the output end of the current amplifier 16 and the consltant-current source 26 is connected to the other input to the comparator 18, and the comparator 18 changes the duty factor of the control signal, which gives the ratio between the ON time and the OFF time of the switching element 7, according to a comparison between them. That is to say, when the potential at the connection point a is 2.0 V, the comparator 18 sets the duty factor of the control signal at 0%. When the potential at the connection point a is 3.0 V, the comparator 18 outputs a control signal whose waveform has a duty factor of 100%.
FIG. 4 shows the characteristic of the comparator 18. As shown in FIG. 4, as the potential at the connection point a varies in the range of 2.0 V to 3.0 V, and therefore as the potential A.sub.OUT at the output end of the current amplifier 16 varies in the range of 1.4 V to 2.4 V, since the forward drop voltage of the diodes 27 and 28 are both 0.6 V, then the comparator 18 outputs the control signal having a waveform whose duty factor linearly changes in the range of 0% to 100%. In this case, when the duty factor is 0%, the ratio of the ON time of the switching element 7 to one period of the output wave of the oscillator 17 is 0%, and when the duty factor of the control signal is 100%, the ratio of the ON time of the switching element 7 is 100%.
In FIG. 3, (B) is a diagram showing the waveform of the potential at the connection point a exhibited when the load 10 is in the heavy load condition, with respect to the triangular voltage waveform of the oscillator 17 shown in (A) of FIG. 3. That is to say, (B) in FIG. 3 shows the potential waveform at the connection point a exhibited when the triangular wave of the oscillator 17 has the peak voltage of 3.0 V, the valley voltage of 2.0 V, and the frequency of 20 kHz.
As shown in (B) of FIG. 3, since (1) the waveform at one input end of the multiplier 15 captures the AC power-supply voltage through the voltage drop at the resistor 33 and (2) the output waveform of the current detecting resistor 12 is similar to the power-supply waveform of the AC power supply 1, the potential at the connection point a or the output A.sub.OUT of the current amplifier 16 also shows a waveform similar to the full-wave rectified waveform of the AC power supply 1. Accordingly, in the heavy load condition, the peak voltage at the connection point a is 3.0 V and the valley voltage is 2.5 V. Hence the comparator 18 generates the control signal having a waveform with a 100% duty factor when the voltage at the connection point a is at the peak voltage 3.0 V, and it generates the control signal having a waveform with a 50% duty factor when the voltage at the connection point a is at the valley voltage 2.5 V. In this case, as the potential at the connection point a varies in the range of 3.0 V to 2.5 V in the half-wave period of the power-supply voltage (which is 10 ms when the frequency of the AC power supply 1 is 50 Hz), the duty factor of the control signal outputted from the comparator 18 varies in the range of 100% to 50% in this half-wave period (the control signal varies between the ON level and the OFF level at the same frequency as the oscillation output of the oscillator 17). As a result, the ratio between the ON time and the OFF time of the switching element 7 also varies in accordance with the variation of the duty factor of the control signal.
In this way, in the heavy load condition, the period of the ON operation of the switching element 7 is set long so that larger energy can be stored in the reactor 6, and the energy is discharged to the output side in the OFF period of the switching element 7 to increase the charge voltage of the load capacitor 11. This operation controls the output voltage constant in the heavy load condition.
On the other hand, in FIG. 3, (C) is a diagram showing the voltage waveform at the connection point a in the light load or no-load condition is shown. The output waveform of the oscillator 17 varies as shown in (A) of FIG. 3. In this case, both the valley voltage and the peak voltage of the voltage waveform at the connection point a show values slightly larger than 2.0 V. This is because of the following reason. That is to say, when the DC output voltage rises to the set value and the load 10 changes from the heavy load condition to the no-load or light load condition, the output of the voltage amplifier 14 indicates 0 V and then the multiplier 15 operates to minimize its output, which causes the current amplifier 16 to output an output signal at such a level that the voltage at the connection point a becomes as close to the valley voltage 2.0 V of the triangular wave of the oscillator 17 as possible (which is a value near 1.4 V). As a result, the comparator 18 operates to cause the DC output voltage to be constant. However, the duty factor of the control signal does not become perfectly 0% but it attains the minimum duty factor, and the comparator 18 performs ON/OFF operation with this minimum duty factor and outputs the control signal. Since the switching element 7 turns on/off in response to it, a small current is supplied to the load 10 and the load capacitor 11. If the current value is larger than or equal to the current value that the load 10 requires in the light load or no-load condition, the extra current corresponding to the difference keeps charging the load capacitor 11 and the output voltage v.sub.o gradually rises. With this voltage rise, the output voltage rises over the level value at which the load voltage can be controlled constant. If this voltage rise is left as it goes, the output voltage continues rising, and finally the output voltage v.sub.o exceeds the breakdown voltage of the load capacitor 11 and deteriorates the performance of the load capacitor 11. Accordingly, as stated above, when the output voltage becomes a level set as the overvoltage or above, the conventional circuit shown in FIG. 14 detects this state and an external device recognizes the occurrence of the abnormality, and the activation stop command from outside puts the active filter and its control circuit perfectly in the OFF state to prevent the above-described problem in advance.
The above-described function is originally a protective function against abnormal operation. However, in practice, the load 10 is very likely to come in the light load or no-load condition. For example, suppose that the load 10 is an inverter in a household air conditioner; when the temperature in the room which the air conditioner controls becomes lower than the set temperature, for example, the air conditioner stops the compressor and realizes the no-load condition so as to decrease the room temperature. This operation frequently occurs. Recognizing this frequent phenomenon as abnormal operation every time introduces a serious problem. It is therefore desired that the output voltage of the active filter can be always controlled constant even when the load comes in the light load or no-load condition.
As described above, the load 10 frequently comes in the light load or no-load condition. However, in the conventional technique, it is each time recognized as an abnormality and the active filter is completely stopped. This is very inconvenient to use as a control method.
The timing chart of FIG. 15 shows waveforms obtained by reviewing the operation of the individual portions of the conventional circuit shown in FIG. 14. In (a) of FIG. 15, a character v.sub.i shows the voltage waveform of the AC power supply 1. A character i.sub.i in (b) of FIG. 15 shows the current waveform coming from the AC power supply 1. The waveform obtained by full-wave rectifying the AC current i.sub.i is represented as a voltage in the current detector 12. A character v.sub.o in (c) of FIG. 15 shows the output voltage waveform across the output terminals of the active filter. A character ON/OFF in (f) of FIG. 15 shows ON/OFF operation of the start switch 22, and the voltage waveform at the point b in FIG. 14 is shown as the voltage V.sub.b in (d) of FIG. 15. A character V.sub.OUT in (g) of FIG. 15 shows the output waveform of the voltage amplifier 14, and a character A.sub.OUT in (h) of FIG. 15 shows the output waveform of the current amplifier 16.
As shown in FIG. 15, when the start switch 22 operates into the ON state at time t.sub.1, the soft start DC power supply 23 is connected to the diode 27. Then the potential V.sub.b at the connection point b rises as shown in (d) of FIG. 15 and the current flows to the diode 28 from the constant-current source 26 accordingly, and then the comparator 18 and the gate driver 19 start driving and the switching element 7 turns on. At first, the load capacitor 11 is charged with a voltage corresponding to the peak value of the power-supply voltage of the AC power supply 1. When the voltage set value in the voltage setter 13 is sufficiently higher than the peak value of the power-supply voltage of the AC power supply 1, the comparator 18 outputs the control signal whose waveform has a duty factor of almost 100% to turn on the switching element 7. As a result, a large AC current flows and the load capacitor 11 is rapidly charged.
When the charge voltage at the load capacitor 11 reaches the voltage set value at time t.sub.2, the negative feedback control of the control circuit 200P causes the value of the AC power-supply current i.sub.i to rapidly attenuate from the large current value to a current value required by the load 10. At this time, if the load 10 is in the light load condition or the no-load condition, this AC power-supply current is controlled to the minimized value (lower than or equal to about 5% to 10% of the current in the heavy load condition) and the output V.sub.OUT of the voltage amplifier 14 falls from the maximum voltage toward 0 V.
Now, the voltage V.sub.b at the point b shows an AC waveform having its maximum at about 3.0 V (strictly 3.0 V-0.6 V). The output voltage A.sub.OUT of the current amplifier 16 shows an AC waveform having its peak voltage at about 2.4 V which is lower than the potential at the connection point a by the forward drop voltage of the diode 28 (0.6 V). However, since the voltage V.sub.OUT comes to a value close to 0 V as the output voltage v.sub.o reaches the voltage set value at time t.sub.2, the voltage A.sub.OUT changes to a waveform similar to the AC full-wave rectified waveform having its valley voltage at about 1.4 V and the current amplifier 16 operates to minimize the ON time of the switching element 7. As a result, the current flowing to the output terminals of the active filter becomes smaller and the voltage of the load capacitor 11 gradually rises for the reason described above.
When the output voltage reaches the voltage set as the overvoltage at time t.sub.3, the hysteresis comparator 20 operates and the flip-flop 21 operates to output the detect signal F.sub.o to the outside. With the output of the detect signal F.sub.o, the external microcomputer 500 recognizes it as an abnormality and outputs the activation stop command signal OFF to turn off the start switch 22 so as to bring the potential V.sub.b at the point b to the ground potential. This stops the operation of the switching element 7 and the output voltage v.sub.o stops rising. After that, the load capacitor 10 discharges and the output voltage v.sub.o gradually decreases.
When the output voltage v.sub.o reaches the reset voltage of the hysteresis comparator 20 at time t.sub.5, the comparator 20 is reset. After that, as the external microcomputer outputs the activation command signal ON to the start switch 22 again to turn on the switch 22, the main circuit 100 performs the active filter operation again.
However, in this case, the output voltage A.sub.OUT rises toward the original maximum level in time t.sub.3 to t.sub.5, that is, if the output voltage A.sub.OUT has risen to or above the condition in which the output of the comparator 18 shows the 100% duty factor waveform, then a large AC current i.sub.i flows again to the switching element 7 when the start switch 22 is turned on again (time t.sub.5). If the load 10 is in the light condition at this time, the extra large current rapidly charges the load capacitor 11 and the output voltage v.sub.o exceeds the overvoltage set value again at time t.sub.6, and then the hysteresis comparator 20 immediately operates. As the result, the detect signal F.sub.o is outputted from the flip-flop 21 again and the external microcomputer 500 detects this as an occurrence of an abnormality and controls the start switch 22 again to force the potential V.sub.b at the point b to drop to the ground potential, and the active filter 100 stops again for the occurrence of the abnormality. In this period (time t.sub.5 to t.sub.6), the excessive current flows to the reactor 6 as an inrush current, which may cause the reactor 6 to generate abnormal sound.
While this series of operations functions to quickly protect the load capacitor I11 from overvoltage, the procedure of restarting is required every time the active filter 100 and its control circuit 200P stop operation, and the overvoltage state immediately occurs every time the restarting operation is performed.
Furthermore, since this phenomenon occurs in a certain cycle, it is seriously recognized as abnormal sound, and the device is very defective as a product.
The above-described problems become more serious when combined with the following problem.
That is to say, while fixed voltage control may be required for the DC output voltage of the active filter, variable voltage control is increasingly demanded to apply PAM control (Pulse Amplitude Modulation) to an inverter serving as the load of the active filter. This is because the PAM control is enabling remarkable improvement of the efficiency of the entire control system including the active filter, the inverter, and the load motor of the inverter. Thus, (1) the active filter is required to have the function of allowing the DC output voltage to be variably set from outside. Further, it is also strongly demanded that (2) even when the set voltage is changed as stated above, the set voltage should be controlled constant within a given range over the entire load range from the light load or no-load to the rated load.
However, even when the output voltage has a fixed value, the conventional active filter described above cannot control the output voltage within a constant range without causing the above-described problems when the load changes to the light load or no-load condition. That is to say, actually, it is definitely impossible to satisfy the requirements (1) and (2) without encountering the problems.
The problems of the conventional circuit can be summarized as follows:
(First Problem)
First, granting it to be right that the active filter operation is stopped after the output voltage over the overvoltage set value is detected to control the output voltage in the constant range, recognizing the detection as an abnormality every time and stopping and starting the control circuit from outside renders the active filter very inconvenient for users from the viewpoint of controllability. When considering the situation in which the load changes from the heavy load condition to the light load or no-load condition, the phenomenon that the output voltage rises to the level of the overvoltage set value must be regarded as a frequently occurring phenomenon. It must be though over whether it is necessary to recognize this phenomenon as "occurrence of an abnormality" which causes the system to stop. We must change the way of conceiving in this sense.
(Second Problem)
The second problem is relevant to the first problem. That is to say, when the output voltage falls after the operation of the active filter is stopped and reaches the reset voltage, the conventional technique detects the state and restarts the active filter. We should note the problem that an excessive current flows at this time. This causes the reactor to generate abnormal sound. Furthermore, after the restart, the system soon comes in the overvoltage state again, and the system is caught in a vicious circle that the activation must be stopped again.
(Third Problem)
While the protective set value can be a fixed voltage value in respect of protection of the load capacitor from overvoltage, the DC output voltage of the active filter must be set freely to satisfy the demand of applying the PAM control to the load side.
We now consider whether this demand can be satisfied in the conventional circuit shown in FIG. 14. For example, suppose that the overvoltage protective set value has a fixed value of 400 V and the DC output voltage of the active filter can be controlled freely in the range of 200 V to 380 V by changing the set value in the circuit 13 shown in FIG. 14. In this case, when the output voltage is selected to 200 V, for example, and the load rapidly changes from the rated load to the light load or no-load, and then the DC output voltage rises from 200 V to 400 V and the overvoltage is detected. In this case, the control range of the output voltage is 200 V. For another example, when the set value of the DC output voltage is 300 V, the DC output voltage rises from 300 V to 400 V and the overvoltage is detected. In this case, the control range of the DC output voltage is 100 V. Such control is far from being a DC output type constant control.
As stated above, the conventional technique cannot solve even the third problem.